Automatic control system

The distributed control system (DCS) of an organic fertilizer plant is a computer control system used in industrial production processes. Its control units are scattered throughout the system to achieve "decentralized control and centralized management".

In organic fertilizer production, DCS collects key parameters such as fermentation tank temperature (accuracy up to ±1°C), material flow (control error ±0.5%), and granulator speed through sensors in real time, and transmits the data to the central control station. For example, in the composting fermentation process, the system can automatically adjust the frequency of the turning machine according to the real-time monitored temperature and humidity, so that the fermentation temperature is stabilized in the optimal range of 55-65°C to ensure microbial activity and composting effect. In the granulation process, the granule qualification rate can be increased to more than 98% by accurately controlling the extrusion pressure and die speed.

The operator can monitor and operate the entire production line uniformly through the human-machine interface (HMI) in the central control room, such as adjusting the hot air flow of the dryer and controlling the metering accuracy of the packaging machine (error ±0.2kg). At the same time, DCS has a redundant design, and key control modules and communication networks are equipped with backup units. When local equipment fails, the backup unit can be seamlessly switched within 1 second to ensure production continuity. In addition, the system can also record more than 3 years of production data, optimize process parameters through data analysis, and improve production efficiency and product quality stability.

In the control system, DCS is the abbreviation of Distributed Control System. It integrates computer technology, control technology, communication technology and graphic display technology. It is an automation system that disperses control functions and centralizes operation management.

In actual applications, the DCS system collects data in real time through various sensors (such as temperature, pressure, and flow sensors) distributed at the production site, with a collection frequency of more than 10 times per second. These data are transmitted to the control station through the communication network for processing.

The control station uses advanced control algorithms (such as PID control) to calculate the data, and then sends the control instructions to the actuator to achieve precise adjustment of the production process. For example, in chemical production, the temperature control accuracy can be maintained at ±1℃, and the flow control accuracy can reach ±0.5%.

At the same time, the operator can monitor the entire production process centrally through the human-machine interface at the operation station in the central control room. The engineer station supports operations such as programming and configuration modification of the system. In addition, the DCS system adopts a redundant design, and key equipment (such as controllers and communication networks) are backed up, and the fault switching time is less than 1 second, ensuring stable and reliable operation of the system.

DCS (distributed control system) and SCADA (data acquisition and monitoring control system) are two common systems in the field of industrial automation, but there are many differences. From a functional point of view, DCS focuses on closed-loop control of the production process, and realizes real-time control of field equipment through control stations.

For example, in compound fertilizer production, it can control the raw material ratio of the batching scale with an accuracy of ±0.2%, and control the temperature fluctuation of the dryer within ±2℃ through the PID algorithm. SCADA emphasizes data collection and monitoring. It can collect data over a large geographical area. For example, in oil and gas pipeline monitoring, the SCADA system can collect pressure, flow and other data along the line at a frequency of more than 10 times per second, but the realization of control functions is relatively weak.

From an architectural point of view, DCS adopts a distributed architecture, and the control functions are scattered in various control stations. Even if some stations fail, the system can still maintain operation, and the redundant switching time of its control station is usually less than 100 milliseconds;

SCADA is mostly a centralized management architecture, mainly composed of a host computer, a communication network and a remote terminal unit (RTU), and pays more attention to data transmission and centralized display. In addition, DCS is often used in continuous production processes, such as in the chemical and electric power industries; SCADA is widely used in scenarios with high data monitoring requirements, such as urban water supply and rail transit.

PLC (Programmable Logic Controller) and DCS (Distributed Control System) are both commonly used control systems in the field of industrial automation, but there are significant differences between the two. From the perspective of control scale, PLC is usually suitable for small, local control scenarios, such as Siemens S7-200 SMART series PLC with I/O points ranging from 0 to 256 points for a single device or a small production line; while DCS is oriented to large and complex systems and can manage thousands of control nodes, such as Honeywell's PlantCruise system, which can easily handle more than 10,000 control points.

In terms of functional characteristics, PLC is mainly based on logic control, good at sequential control and switch quantity control, fast response speed, and the scanning cycle can reach milliseconds; DCS emphasizes more on the precise control of analog quantities and multi-variable coordination. For example, in chemical production, the control accuracy of parameters such as temperature and pressure can be maintained within ±1%.

From the perspective of architectural design, PLCs are mostly centralized or small distributed, with relatively simple system structures, and are often used for single-machine equipment control in automated production lines; DCS uses a distributed architecture, consisting of control stations, operating stations, etc., and realizes decentralized control and centralized management through the network, which is suitable for comprehensive monitoring across regions and multiple devices. In addition, DCS has more powerful data processing, storage and analysis capabilities, and can meet the complex management needs of large-scale production processes, while PLC focuses on local control of equipment and simple data interaction.

The controller used in the DCS distributed control system is the process control unit PCU, also known as the main control unit or control station. This type of controller usually adopts a redundant design to ensure the high reliability of the system, and the redundant switching time of key components is generally less than 10 milliseconds.

In terms of performance, the process control unit has powerful computing power and can support a variety of control algorithms, such as proportional integral differential PID control, cascade control, feedforward control, etc. Generally speaking, it can process thousands of control loops at the same time, with a scan cycle as short as tens of milliseconds to quickly collect and process data. When processing analog signals such as temperature, pressure, and flow, the sampling accuracy can reach 16 bits or even higher.

The controller also supports a variety of communication protocols, such as Modbus, Profibus, OPC UA, etc., and can interact efficiently with field instruments, actuators, and host computers. In the production process of compound fertilizers, it can accurately control the weighing accuracy of the batching scale (the error is controlled within ±0.2%), the speed and temperature of the granulator (temperature control accuracy ±2℃), etc., to achieve stable and efficient operation of the entire production process.

Control station‌: This is the core part of the DCS system, responsible for collecting signals from field equipment, collecting the operating parameters of field equipment in real time through various sensors (such as temperature sensors with an accuracy of up to ±0.5℃, pressure sensors with an accuracy of ±0.2% FS), and processing data according to the preset control strategy, and then outputting the control signal to the field equipment to drive the actuator (such as regulating valve, frequency converter) to perform closed-loop control on the fan speed, granulator speed, etc. The control cycle is usually between 50ms - 1000ms.

‌Operation station‌: The operation station is the interface of human-computer interaction. The operator can monitor the operating status of the industrial process, set and modify parameters, and issue control instructions through the operation station. tongli uses a high-resolution display (common resolution 1920×1080) to monitor the production process in real time, and can perform operations such as parameter setting and equipment start and stop. The data refresh rate is generally less than 1 second.

The operation station is a human-computer interaction interface. The operator uses the engineer station for system development and maintenance. It supports control program writing and system configuration and can quickly locate the fault point. The communication network is responsible for data transmission of various parts of the system. It adopts redundant design and the transmission rate can reach more than 100Mbps.

‌On-site sensor: Field equipment includes various sensors, actuators, etc., which are directly connected to production equipment, responsible for collecting the actual parameters of the industrial process, and transmitting these parameters to the control station, while receiving the control signal of the control station and performing corresponding actions.

The use of the DCS system can accurately control each link of the fertilizer production line through automation, automatically adjust equipment parameters, collect and analyze data in real time, ensure accurate raw material ratio, stable process parameters, ensure consistent product quality, and monitor safety hazards in real time; without the use of the DCS system, relying on manual operation can easily lead to low production efficiency, inconsistent production rhythm, and inaccurate parameter control, resulting in large fluctuations in product quality.

Control accuracy: The DCS system can achieve precise control. In the compound fertilizer batching link, the error of raw material ratio can be controlled within ±2%, while the error of manual control or traditional simple control system may reach ±2%, resulting in large fluctuations in product nutrient content.

Production stability: The DCS system can collect production data in real time, respond quickly and adjust equipment operation. For example, the temperature of the dryer, DCS can control the fluctuation range within 2℃, without the DCS system, the temperature fluctuation may exceed 10℃, affecting the quality of the particles.

Fault handling efficiency: The DCS system has fault diagnosis function, and the average fault location time is only 1-2 minutes. It can quickly issue an alarm and record fault information, and the average fault repair time (MTTR) is about 3 minutes. Without the DCS system, fault troubleshooting and handling take a long time.

Production efficiency: The DCS system realizes automated continuous production, which can increase the efficiency of compound fertilizer production lines by 30% - 50%. For example, in the packaging and palletizing process, the packaging machine and palletizing machine under DCS control can cooperate to pack and palletize 800-1000 bags per hour, and the manual operation efficiency is much lower than this.

Data management: The DCS system can collect data more than 10 times per second, establish a real-time database, and update the cycle less than 1 second, which is convenient for production data traceability and analysis. Without the DCS system, data recording is difficult, and it is difficult to carry out effective production optimization.

System scalability: The DCS system adopts a modular design. When adding new equipment or adjusting the process, it can be completed through simple configuration modifications and a small amount of hardware addition. However, for production lines without a DCS system, equipment expansion or process adjustment often requires large-scale transformation, which is costly and time-consuming.

Centralized control is centered on a single central controller, where all device data is aggregated for processing and decision-making. It is suitable for small and simple systems such as BB fertilizer production lines. However, in large and complex scenarios, processing delays are prone to occur due to data overload, and once the central controller fails, the entire system will be paralyzed. The mean time between failures (MTBF) is about 5,000 hours, and the scalability is poor. When adding new equipment or functions, the core controller needs to be significantly modified. Distributed control (DCS) consists of multiple decentralized control stations. Each station processes local data independently and only exchanges key information. The response speed is improved to a few hundred milliseconds compared to centralized control. This decentralized design is somewhat similar to Bitcoin. With redundant design, its MTBF can reach more than 10,000 hours, and local failures do not affect overall operation. System expansion can be achieved by flexibly adding and reducing control stations, which is more in line with the dynamic changes in large and complex industrial systems and represents the mainstream direction of modern industrial automation control.